1. Field of the Invention
The present invention relates to a method of determining a pitch arrangement of a tire. More particularly it relates to a method which makes it possible to efficiently and easily determine the pitch arrangement for attaining an improved noise performance of a tire, as well as to determine the best pitch arrangement of the tire at economic cost.
2. Description of the Prior Art
Design work of a vehicle tire running safely and quietly at a high speed on a dry, wet and an ice and snow road has been well understood, and the consideration of a tire which runs quietly has been done theoretically. Grooves arranged on a tread of the tire are designed based on a repeated design cycle of a plurality of variable pitches according to a formula calculated mathematically.
Land portions (called design elements hereinafter) are divided by lateral grooves and circumferential grooves, which form the pitch and the pitch arrangement on the tire surface, and are arranged by a manual operation or a tire mold based on a design value. The term pitch means a relative length of the design element, and the pitch arrangement means an order of the pitch arranged on the tire surface.
As described in Japanese Patent Laid-Open 4-232105, though pitches can be different lengths, less than approximately 9 kinds or less are used in practice. A specific pitch length of a certain pitch arrangement can be varied based on the circumferential length of the tire. As a result, when a second tire runs at a lower speed than a first tire, the specific pitch arrangement of the pitch having a relative length of the first tire whose diameter is greater than that of the second tire generates the same acoustic fingerprint as that of the second tire. (It can be simulated by a String Plot which shows characteristics. The String Plot plots amplitude&lt;ordinate&gt; against harmonic number&lt;abscissa&gt; which shows the number of the times of vibration during one round when the tire runs with noise.)
A method of design of the specific tread is described by Landers in U.S. Pat. No. 4,327,792 and U.S. Pat. No. 4,474,223. Another approach to optimize the design of the tread is described by Flechtner in U.S. Pat. No. 4,598,747. In this way, slits are formed on each projection, and a wave length and an amplitude of the slits to a dimension of the slits are decreased as much as possible while satisfying other standards. Further, Compos et al describe a mathematical method of the design in U.S. Pat. No. 4,598,748.
In order to determine whether or not the above mentioned approaches to the design of the quiet tire succeed, a test must be done for the tire, and the tire furnished with enough dimension to be tested must be physically manufactured. In general, the test of the tire for "noise level and quality" is done by adopting four (or less) experimental tires to wheel rims of a car driven by a person having sensitive ears.
The best way to test a noise characteristic of the tread design conveniently with high reliability is to simulate the tread noise of the tire running on a road surface by a computer, as described-in detail by Parker et al in U.S. Pat. No. 4,727,501. Japanese Patent Laid-Open 4-232105, teaches a way to determine a ratio of the pitch length (pitch ratio) to be white noise based on prime number.
In any of the above mentioned ways, however, because a combination of the pitch arrangement which provides the quiet tire and the best pitch ratio might be missed, only a limited number of the pitch ratios and the pitch arrangement can be designed and tested.
It is an object of the present invention to provide a method of determining a pitch arrangement which decreases the possibility of avoiding the combination of the best pitch as far as possible.